A method for suppressing combustion noise and NOx emissions from a diesel engine by performing a pilot injection operation is known. The pilot injection operation includes injecting a small amount of fuel into a combustion chamber prior to a main injection operation. Since pilot injection operations typically include a small quantity of fuel, which is referred to as a command quantity, it is desired to improve the accuracy of the quantity injected to obtain sufficient effects, namely, reduction of combustion noise and NOx emission. One way to improve accuracy is to implement an injection quantity learning process. The process includes determining a difference between an actual quantity of fuel injected (hereinafter referred to as “actual injection quantity”) and the command quantity. The process then compensates for the detected difference.
Thus, the present application provides a fuel injection controller for performing a highly accurate injection quantity learning process, as disclosed in unpublished Japanese Patent Application No. 2003-185633, which corresponds to U.S. Pat. No. 6,907,861. The fuel injection controller is adapted such that during an idle state while no fuel is injected and the engine is running a one-shot injection for learning is performed for a particular one of a plurality of cylinders of a diesel engine. An idle state is present when the command quantity of fuel for the injectors is not greater than zero, for example, during a gear change and deceleration of the vehicle. The controller determines the actual injection quantity based on a variation in engine speed caused by the one-shot injection. The controller then corrects the command quantity depending upon a difference between the actual injection quantity and the command value for the performed one-shot injection.
To increase the accuracy of the correction of the command value in the above-described injection quantity learning process, it should be arranged such that a characteristic (e.g., the variation in engine speed, air-fuel ratio, or pressure in the cylinder) representative of an effect of the one-shot injection operation can identify different values of the actual injection quantity. More specifically, a value of the characteristic should correspond to a particular value of the actual injection quantity in a one-to-one relationship. Therefore, different values of the characteristic should never be obtained where the one-shot injection is performed a plurality of times under the same conditions. Conversely, a same value of the characteristic should never be obtained where the one-shot injection is performed a plurality of times under different conditions.
Where the characteristic is engine speed variation, the actual injection quantity and the characteristic do not have a one-to-one relationship and are roughly classified into the following two categories:
a) Combustion varies although the injection quantity is constant.
The value of the characteristic (hereinafter referred to as the “characteristic value”) varies depending upon whether or not a sufficient amount of oxygen for the complete combustion of the injected fuel is provided. In addition, the combustion is slowed when the exhaust gas is recirculated by an EGR system, thereby varying the detected characteristic value.
b) An engine load varies during the detection of the characteristic value.
A pumping loss or energy loss in compressing the intake air, and other changes occur when the intake airflow is varied, thereby affecting the characteristic value.
Thus, to have the actual injection quantity and the characteristic value in a one-to-one relationship, the airflow into the combustion chamber should be controlled. The unpublished Japanese patent document described above, however, does not teach details of such a control of airflow and, thus, there may be a case when the detected characteristic value and the actual injection quantity do not correspond in a one-to-one relationship.
The present invention has been developed in view of the above-described situations to provide an injection quantity control apparatus for a diesel engine that establishes a suitable learning environment when injection quantity learning is to be performed. This ensures that the characteristic value is in a one-to-one relationship with the actual injection quantity, thereby enabling a highly accurate injection quantity learning process to be performed.